focal (8) smrsh.8.gz

NAME

       smrsh - restricted shell for sendmail

SYNOPSIS

       smrsh -c command

DESCRIPTION

       The  smrsh  program  is  intended  as  a replacement for sh for use in the ``prog'' mailer in sendmail(8)
       configuration files.  It sharply limits the commands that can be run using  the  ``|program''  syntax  of
       sendmail  in  order  to improve the over all security of your system.  Briefly, even if a ``bad guy'' can
       get sendmail to run a program without going through an alias or forward file, smrsh  limits  the  set  of
       programs that he or she can execute.

       Briefly,  smrsh  limits  programs  to  be in a single directory, by default /etc/mail/smrsh, allowing the
       system administrator to choose the set  of  acceptable  commands,  and  to  the  shell  builtin  commands
       ``exec'',  ``exit'',  and ``echo''.  It also rejects any commands with the characters ``', `<', `>', `;',
       `$', `(', `)', `\r' (carriage return), or `\n' (newline) on the  command  line  to  prevent  ``end  run''
       attacks.   It  allows  ``||''  and ``&&'' to enable commands like: ``"|exec /usr/local/bin/filter || exit
       75"''

       Initial   pathnames   on   programs   are   stripped,    so    forwarding    to    ``/usr/ucb/vacation'',
       ``/usr/bin/vacation'',  ``/home/server/mydir/bin/vacation'',  and  ``vacation''  all  actually forward to
       ``/etc/mail/smrsh/vacation''.

       System administrators should be conservative about populating  the  sm.bin  directory.   For  example,  a
       reasonable  additions  is vacation(1), and the like.  No matter how brow-beaten you may be, never include
       any shell or shell-like program (such as perl(1)) in the sm.bin  directory.   Note  that  this  does  not
       restrict  the  use  of shell or perl scripts in the sm.bin directory (using the ``#!'' syntax); it simply
       disallows execution of arbitrary programs.  Also, including mail filtering programs such  as  procmail(1)
       is a very bad idea.  procmail(1) allows users to run arbitrary programs in their procmailrc(5).

COMPILATION

       Compilation  should  be trivial on most systems.  You may need to use -DSMRSH_PATH=\"path\" to adjust the
       default search path (defaults to ``/bin:/usr/bin:/usr/ucb'') and/or -DSMRSH_CMDDIR=\"dir\" to change  the
       default program directory (defaults to ``/etc/mail/smrsh'').

FILES

       /etc/mail/smrsh - default directory for restricted programs on most OSs

       /var/adm/sm.bin - directory for restricted programs on HP UX and Solaris

       /usr/libexec/sm.bin - directory for restricted programs on FreeBSD (>= 3.3) and DragonFly BSD

SEE ALSO

       sendmail(8)

                                          $Date: 2013-11-22 20:52:00 $                                  SMRSH(8)